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The Middle Cambrian Fossil Pikaia and the Evolution of Chordate Swimming Thurston Lacalli*

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The Middle Cambrian Fossil Pikaia and the Evolution of Chordate Swimming Thurston Lacalli* Lacalli EvoDevo 2012, 3:12 http://www.evodevojournal.com/content/3/1/12 COMMENTARY Open Access The Middle Cambrian fossil Pikaia and the evolution of chordate swimming Thurston Lacalli* Abstract Conway Morris and Caron (2012) have recently published an account of virtually all the available information on Pikaia gracilens, a well-known Cambrian fossil and supposed basal chordate, and propose on this basis some new ideas about Pikaia’s anatomy and evolutionary significance. Chief among its chordate-like features are the putative myomeres, a regular series of vertical bands that extends the length of the body. These differ from the myomeres of living chordates in that boundaries between them (the myosepta) are gently curved, with minimal overlap, whereas amphioxus and vertebrates have strongly overlapping V- and W-shaped myomeres. The implication, on biomechanical grounds, is that myomeres in Pikaia exerted much less tension on the myosepta, so the animal would have been incapable of swimming as rapidly as living chordates operating in the fast-twitch mode used for escape and attack. Pikaia either lacked the fast-twitch fibers necessary for such speeds, having instead only slow-twitch fibers, or it had an ancestral fiber type with functional capabilities more like modern slow fibers than fast ones. The first option is supported by the sequence of development in zebrafish, where both myoseptum formation and fast fiber deployment show a dependence on slow fibers, which develop first. For Pikaia, the absence of fast fibers has both behavioral and anatomical implications, which are discussed. Among the latter is the possibility that a notochord may not have been needed as a primary stiffening device if other structures (for example, the dorsal organ) could perform that role. Keywords: Pikaia, Basal chordates, Myomere and notochord evolution, Amphioxus, Swimming mechanics, Vertebrate muscle fibers, Yunnanozoans Background muscles, and the authors identify an axial trace that Zoology texts typically list four diagnostic features of could be either a notochord or a notochord and nerve chordates: pharyngeal (that is, gill) slits or pores, a noto- cord combined. In addition, however, there are peculiar chord, a dorsal nerve cord and serial (or segmental) features not known from living chordates: a sausage- muscles. This last feature, represented by the somite- shaped dorsal organ running the length of the trunk, derived myomere series in the case of cephalochordates and an anterior shield-like structure, the anterior dorsal (amphioxus) and vertebrates, is the subject of this ac- unit, covering the head region. Pikaia does not, there- count, stimulated by the recent description [1] by Simon fore, fit entirely comfortably with modern chordates, Conway Morris and Jean-Bernard Caron (here referred suggesting that it is either divergent, if it is a chordate, to as CMC) of the Middle Cambrian fossil Pikaia graci- or is a basal member of the chordate lineage differing in lens (Figure 1) from the Burgess Shale of British significant ways from surviving members of that lineage. Columbia. The authors interpret Pikaia as a basal Of the chordate features listed above, the first, the chordate and, though this conclusion is provisional, it pharyngeal slits, have an evolutionary history that prob- would be perverse to deny the key similarities between ably predates chordates by a considerable interval, be- this animal and what would be expected of a basal cause apparently homologous structures occur in more chordate: much of the body is occupied by a series of basal deuterostome phyla, in living hemichordates and vertical bands resembling the septa between segmental fossil echinoderms [2-4]. Pharyngeal pores or slits, where they occur, are assumed to play an ancestral role in Correspondence: [email protected] deposit- or filter-feeding as a means for disposing of ex- Biology Department, University of Victoria, Cunningham Building, Victoria, cess water entering the mouth and pharynx with food BC V8W-3N5, Canada © 2012 Lacalli; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Lacalli EvoDevo 2012, 3:12 Page 2 of 6 http://www.evodevojournal.com/content/3/1/12 overlap between adjacent myomeres nowhere exceeds more than about one segment. CMC discuss this in relation to the much more angled V- and W-shaped myomeres of amphioxus and vertebrates, but there is more to be drawn from this comparison in my view. Tunicate larvae are omit- ted from both CMC and this account because, based on current phylogenies [6], both they and their tail muscula- ture are reduced secondarily to a degree that makes them uninformative regardingthenatureofancestral myomeres. Vertebrates, as one would expect, have the most compli- cated segmental architecture. Myomeres in adult fishes are substantial masses of tissue whose thickness seriously com- plicates the task of analyzing the internal stresses generated during swimming. This is chiefly because the tension Figure 1 Pikaia gracilens, as reconstructed by Conway Morris and Caron [1].The head bears a pair of tentacles, probably sensory exerted on the spinal axis as a given fiber contracts varies in nature, and paired rows of ventrolateral projections that may be with the distance that fiber lies from the central axis. To gills. Not shown: the expanded anterior (pharyngeal) region of the avoid producing unequal stresses across the myomere, the digestive tract, and the dorsal shield-like structure, the anterior fibers are arranged helically and the myosepta have a com- dorsal unit, that lies above it. The boxed detail shows the main axial plex three-dimensional shape approximating a W [7,8]. The features: the dorsal organ (do), and the putative notochord (not) and digestive tract (dt). The size range among specimens is 1.5 to accepted explanation, which requires some fairly sophisti- 6 cm, which makes this animal very close in size to the adult stage cated modeling [9], is that this arrangement minimizes the of modern lancelets (amphioxus). difference in tension experienced on the opposing faces of each myoseptum as a means of reducing energy loss. Myomeres in larval fish and in amphioxus are chevron- particles. The remaining three features mentioned above, or V-shaped and are much less massive. They are flattened which together constitute the axial complex, are respon- mediolaterally and so, in effect, can be treated as a first ap- sible for undulatory swimming. These are considered to proximation as flat, two-dimensional structures. This be restricted to chordates alone, and form a suite of makes it much easier to understand the biomechanical characters that are closely linked anatomically, and that issues involved without resorting to a detailed mathemat- operate functionally as a unit [4]. Of the three, only the ical and computer analysis. Muscle fibers in amphioxus at- notochord has a plausible counterpart, the enteropneust tach to the myosepta directly [10] rather than to the stomochord, in more basal deuterostomes, but the two notochord sheath, so the greatest stresses on the noto- structures are not currently considered to be homolo- chord are borne where the septa join the sheath. But the gous [5]. We thus have very little evidence from com- whole complex is also bound tightly together by sheets of parative studies to indicate how the axial complex basal lamina, and therefore acts as a single unit, as evolved, all at once or step by step, and if the latter, what opposed to relying on an array of subsidiary tendons and the key steps might have been. CMC have made a start, bony struts to distribute the forces, as in fish. So, using in their paper, by discussing the role the dorsal organ amphioxus as a model, what are the advantages of having may play in antagonizing the force of muscle contraction V-shaped myosepta rather than vertical ones? The key during swimming, at a stage in chordate evolution when seems to be in the angle of inclination, that is, the steep- the notochord might not yet have fully taken over this ness of the V. Because the muscle fibers are aligned hori- function. This commentary on their paper is an attempt zontally, along the body axis, it is in that direction that to carry the argument somewhat further, using simple force is exerted during contraction. Expressed as vectors biomechanical principles to clarify the relation between (Figure 2A), this force has two components, one directed myomere morphology and swimming mode, and to dis- perpendicular to the anterior surface of the myomere (P cuss some of the implications this has for the behavior in the figure), the other parallel to it (along the dashed and mode of life of Pikaia and its kin. line). The former is the more important component, as stresses in that direction are the ones most likely to result Myomere shape and its implications in mechanical failure of the septa, for example, by tearing CMC discuss myomere shape at some length because of the myomeres apart, whereas forces acting parallel to the significant ways this differs between Pikaia and modern septa would only be important if damage by shear were chordates. Myomeres in Pikaia are taller and narrow (box, the key issue. From simple trigonometry, the perpendicu- Figure 1), while the myosepta, which form the interface be- lar component of force is less than the total force of con- tween them, are gently curved, or sigmoidal, such that the traction in proportion to the cosine of the angle the Lacalli EvoDevo 2012, 3:12 Page 3 of 6 http://www.evodevojournal.com/content/3/1/12 septa attaching to it (Figure 2B). This evens out the stress due to contraction. In amphioxus, the overlap ranges from one to two segments in larvae to three to four in the adult, considerably more in the latter than in Pikaia for an animal of approximately similar size.
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